The invention resides in a method of galvanically forming structured plate-like bodies which are structured at one side such that the structures project from a base representing a contiguous electrically conductive surface to which the structure tops are parallel wherein the base is galvanically covered by a metal.
In microstructuring techniques, it is often necessary to galvanically form, by metal deposition, a structure from a plastic plate provided with micro structured bodies by means of microforming techniques or X-ray depth lithography. The microstructured bodies on the plastic plate are formed in such a way that the structure base, that is the surface from which the microstructures rise, is electrically conductive. The easiest way to provide such an electrically conductive surface is to attach the plastic plate to a metal plate and to remove plastic material around the microstructures to be formed down to the metal plate so that the microstructures are disposed directly on the metal plate and the exposed portions of the metal plate form the structure base. Negative shapes of the microstructured bodies can then be formed galvanically in a galvanic bath in which the metal plate is used as a cathode. In this process, first the interstices between the structured bodies are filled from the structure base on. Upon continuation of the galvanic process, the micro structured bodies are finally covered with metal as the nonconductive microstructured body front surfaces are overgrown. At the end of the process, the microstructured bodies are totally embedded in the metal. Such a process is disclosed for example in U.S. Pat. No. 5,073,237. A process for the galvanic forming of plate-like bodies provided with microstructures is disclosed in this patent. The process described therein serves to provide negative forms of microstructured plate-like bodies which can serve as galvanic molds and whose structure base is a contiguous surface covered with a layer of an electrically conductive material, wherein the electrically conductive material layer is used as a cathode in the subsequent galvanic forming step.
The process utilizes a thermoplastic material layer on which a film of an electrically conductive material is deposited. A microstructured mold insert is impressed into the thermoplastic material layer through the electrically conductive material and is then again removed. With this process, a continuous electrically conductive structure base is formed. During impression of the mold insert into the film-covered thermoplastic material layer, the film ruptures where it is engaged by the micro structures of the mold insert. Upon removal of the mold insert, small, isolated spangles of the film material, which are electrically insulated from one another remain on the vertical wall portions and on the front faces of the microstructures. On the structure base of the negative form however, the film remains undisturbed. With regard to the spangles remaining on the micro structure surfaces of the negative form, it is pointed out that, because of their isolated arrangement, they are electrically insulated from the structure base and therefore, do not prevent exact galvanic forming from the negative mold.
The purpose of this method resides in the formation of a contiguous, electrically conductive structure base which is fully covered by the film of electrically conductive material. The isolated spangles of the film occur as side effects and are therefore--dependent on the thickness of the film and on the shape of the micro structures--distributed on the microstructures in an incidental and non-reproducible manner.
In accordance with the Abstract of J. P. Kokei No. 53-106 643, a plastic substrate is, by means of a molding tool, so structured that concave depressions and convex structure bodies are generated. The plastic substrate is subsequently coated over its whole surface with a thin layer of an electrically conductive material. Then, only those parts of the electrically conductive material are removed which are disposed on the front faces of the convex structure bodies. Subsequently, the concave depressions are galvanically filled with the electrically conductive material. The electrically conductive material projecting beyond the from surfaces of the convex structure bodies is then removed. Since the galvanically deposited material and the convex structure bodies are to have the same height, no problems are encountered by the overgrowth of the galvanic deposits over the front faces.
DE 34 42 781.C2 discloses a method of producing an adjustment disc for cameras wherein a regular relief structure is formed on the top surface of a metal plate by mechanical treatment thereof. The relief structure is then subjected to normal galvanic treatment wherein an emulsion is added to the galvanizing bath which, with a local and timely statistical distribution, inhibits the galvanic deposition and forms the pattern obtained thereby, onto an optical material. With the known method, a metal is galvanically deposited on the whole mechanically structured metal plate, the structure base as well as the structure bodies are electrically conductive. The emulsion inhibits the galvanic deposition on small statistically distributed spots.
The publication Patent Abstracts of Japan, C-861, Aug. 23, 1991, Vol. 15, No. 332 Abstract of JP 3-126,887(A)! describes a method wherein metal powder such as nickel powder, is admixed to a liquid resin. The resin with the metal powder is then filled into a negative form and is cured. The surface of the cured resin is then ground so that a smooth surface is generated on which the powder is exposed. The mold is then placed in a galvanic cell wherein nickel is deposited on the surface of the mold in order to provide a space for the galvanic deposition. In this process, the form does not need to be directly polished since, in this case, the resin is made electrically conductive by the addition of a metal powder. Relatively high concentrations of metal powder have to be used which noticeably change the properties of the resin.
DE-OS16 21 034 discloses a method of making a mold for the galvano-plastic manufacture of sieves, cuttable foils, filters, grids, or similar articles wherein a light sensitive photo layer is disposed on a polished, cleaned metal plate and is exposed to light with the desired pattern. After being developed, the areas exposed to light are covered and the non-exposed areas are dissolved and the metal plate is then etched at the exposed areas to form depressions. According to this known process, the metal plate with the etched depressions from which the exposed photolayer has been removed, is covered and the etched depressions are filled with a hardenable electrically conductive plastic. After hardening, the metal plate and the layer of electrically conductive plastic, not the metal plate, is used for the galvanic molding procedure.
The plastic layer is electrically conductive at all structure areas. The process is not concerned with the galvanic molding.
The present invention is concerned with the problem of evenly growing galvanically deposited material over an electrically non-conductive surface as it is present in the initially referred to normal case on the front surface of structured plastic bodies. The galvanic overgrowth over the front surface of the structure bodies starts at an electrically conductive base, which, as initially mentioned, is slowly built up and fills the depressions or cavities until the microstructure bodies are overgrown and embedded.
This galvanic procedure involves two different processes: the vertical growth in which deposition of the material to be galvanically deposited occurs by way of an electrically conductive metal layer serving as a cathode or on the metal already deposited on the metal layer, and the lateral growth which occurs when the level of the front surface of the microstructures has been reached whereby those front surfaces are finally fully covered with metal.
The speed of the lateral growth under the usual galvanization conditions (nickel sulfate electrolyte, current density 1A/dm.sup.2) is normally on the same order as the vertical growth, that is, it is about 12 .mu./h. During the galvanic overgrowth of stepped structured bodies, it is even possible that faults occur at the level of the intermediate and the upper front surfaces when the galvanic growth fronts meet.
Basically, it would seem helpful to apply to the front faces of the structured bodies a continuous conductive layer which is isolated from the structure base. In this manner, a lateral growth could be achieved with increased speed with regard to the vertical growth. In this case, a sudden contact of the adjacent front faces with the metal grown between the micro-structured bodies would render the whole front face areas cathodic. However, since the rate of metal deposition between the structured bodies is uneven, the formation of such sudden contacts may have the result that further vertical growth in the areas where the galvanic growth has not yet reached the front faces of the structured bodies is interrupted by the galvanic lateral growth. The result will then be that some of the particular structured bodies are not formed and are missing in the galvanically deposited body. At these locations, the galvanically formed product will have cavities. This effect will be greater for larger aspect ratios (relation of height to the width of the bodies) of structured bodies and for greater aspect ratio differences in a particular micro-structure.
It is the object of the present invention to provide a method with which the problems related to the cross-sectional growth of galvanically deposited metal structures are prevented.